This application is related to co-pending, commonly assigned application Ser. No. DP-300043, filed May 23, 2000 for PROCESS FOR FORMING STEEL ROLLER BEARINGS, the disclosure of which is incorporated herein by reference.
The present invention relates to cold formed steel parts and, more particularly, to cold formed high-load bearing steel parts such as roller bearings. Most particularly, the present invention relates to the use of high strength, high carbon steels to form these parts, and to a process for forming them.
High-load bearing steel parts made of high strength steels are widely used in the automotive industry for a variety of applications, such as for example, roller bearings included in hydraulic valve lifters and roller finger followers for automotive engines. Critical features of these roller bearings, such as their inside diameters, outside diameters and diametral surfaces, must be manufactured to close engineering tolerances to optimize performance and to minimize wear under extreme engine operating conditions. The need for high strength steels manufactured to exacting tolerances has become even greater with the application of smaller, more efficient engines in today""s vehicles.
Typically, roller bearings are fabricated from high carbon steel by hot or warm forging processes or by machining bar stock. Hot forging requires first heating a steel slug in its preformed state to a temperature between 1600xc2x0 F. to 2000xc2x0 F. to allow formability. Mechanically forming techniques such as upsetting, heading, and extrusion, which are well known in the art, are then used to bring the shape of the part close to the final desired form. However, because of surface scaling resulting from the high temperature preheating process and because of the dimensional growth of the part as the part cools, a substantial amount of final machining is required to bring the part to its required dimensions. For example, the diametral surfaces of the part must be machined to their final dimensions and surface finish. The inside and outside diameters must also be machined to their final dimensions and surface finish. Because of the amount of material that must be removed, the machining often must be completed in several steps including rough cutting, grinding, and honing. These added machining operations are time consuming, add manufacturing steps to the process, and require the use of expensive machine tools. Furthermore, the added machining operations can produce inconsistent results which can vary significantly from part to part. Moreover, with the use of ovens or heaters to bring the slug up to temperature for forming, and the need to handle extremely hot parts during the process, there is added safety issues.
Warm forging requires the steel slug to be first heated to a lower temperature than the hot forming processxe2x80x94between 300xc2x0 F. and 1600xc2x0 F.xe2x80x94before the part is mechanically formed using one of the forming methods described above. While surface scaling typically does not occur because of the relatively lower temperatures used in the warm forming process, dimensional growth of the part does occur as the part cools. As a result, a substantial amount of final machining is similarly required. Thus, the warm forming process does not eliminate the time consuming and expensive steps of machining and finishing the diametral surfaces and the inner and outer diameters, the dimensional variations caused by the machining and finishing, nor the potential dangers found in the hot forming process.
Machining the part to the required dimensions from bar stock eliminates the expense and potential dangers of having to preheat the slug before forming. But cycle times are even longer than either the hot or warm forming processes since more material has to be removed to reach the final desired dimensions. Moreover, a greater amount of material must be inventoried to manufacture the part and a substantial amount of material is wasted in the form of metal shavings. Expensive machine tools are required and the results from the machining operations can vary widely from part to part. Also, since substantially more machining and final finishing of the part is required, a significant amount of cost is added to the product due to the higher cost of skilled labor and the additional energy consumption associated with the machining and finishing.
Cold forming processes, as defined in this application, are carried out at temperatures ranging from the ambient up to about 300xc2x0 F. and include techniques such as upsetting, heading, and extrusion. U.S. Pat. No. 4,202,082 to Williams discloses a method of manufacturing a spherical bearing rod end having cold formed faces that include an annular area to serve as the outer race member. However, since the method does not use a high strength, high carbon steel slug to form the load bearing surfaces, the finished part must utilize additional metal inserts for the load bearing surfaces. U.S. Pat. No. 5,453,139 to Gallagher, Jr. discloses a method of making cold formed high strength parts from steel that has a ferrite-pearlite microstructure and consists of about 0.30 to about 0.65 wt. % carbon, about 0.30 to about 2.5 wt.% manganese, and up to about 0.35 wt. % of at least one grain refiner from the group consisting of aluminum, niobium, titanium, vanadium, and mixtures thereof, the balance being iron. However, the method is limited to steels having a carbon content lower than the present invention and lower than that found to be most desirable for roller bearings having optimal performance and minimal wear characteristics.
Until the present invention, it was thought that cold forming a part from high strength, high carbon steel would likely result in the formation of cracks in the finished part. Therefore, what was needed in the art is a high load bearing part that can be produced by a method that minimizes the need for finishing and/or machining operations. Furthermore, what was needed in the art is a high load bearing part that does not require additional metal inserts or other parts to serve as the load bearing surfaces. Moreover, what was needed in the art is a high load bearing part that has the preferred carbon content for use in roller bearings. Even further, what was need in the art is a high load bearing part that minimizes the scrap and waste of raw materials. Finally, what was needed in the art is a high load bearing part that can be made to close dimensional tolerances with minimal part-to-part variation.
The present invention relates to a method of forming a high load bearing part, such as a roller bearing, from a high strength, high carbon steel, using a cold forming method of fabricating the part that minimizes waste and minimizes the amount of machining or finishing required after the part is formed. The term xe2x80x9cnet shapexe2x80x9d as used herein means that the dimension of the part, as a result of the forming process, requires no further machining to achieve its final desired dimension.
The present invention provides a high load bearing part made of high carbon steel and a method of forming the part using a cold forming process where its critical surfaces are either net shaped by the cold forming process or require minimal finishing to achieve the desired dimensions.
The present invention is directed to a cold formed high-load bearing steel part comprising high-carbon steel having at least 0.7 weight percent carbon and up to 1.6 weight percent of at least one alloying element. The present invention is further directed to a process for making the steel part that comprises: providing a slug made of high-carbon steel that comprises at least 0.7 weight percent carbon and up to 1.6 weight percent of at least one alloying element and has volume sufficient to form the part; cold forming the slug to form a blank having lateral and end surfaces; and finishing the surfaces as required to form the part.
Further in accordance with the present invention is a process for forming a roller bearing that comprises: providing a cylindrical slug having at least 0.7 weight percent carbon and up to 1.6 weight percent of at least one alloying element and has lateral and circular end surfaces and sufficient volume to form the roller bearing. A centered indentation is formed in each end surface, and a circular chamfer surface is formed at each intersection of the end and lateral surfaces, thereby producing an upsetting slug.
The upsetting slug is cold formed, thereby deepening the centered indentations and forming a blank that has a web separating the deepened indentations and end surfaces and chamfer surfaces that are substantially at net shape for the roller bearing. The web is removed from said blank, thereby forming a pierced blank having a through hole with an inner surface. The inner and lateral surfaces of the pierced blank are finished to dimensions having, respectively, the specified inner and outer diameters for the roller bearing.
The present invention provides a high load bearing part, such as roller bearings made, inexpensively, from high strength, high carbon steels, and using a cold forming process. The high load bearing parts, made according to the present invention can be efficiently made from a steel having a preferred carbon content for improved durability and wear.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of the invention in conjunction with the accompanying drawings.